There is a difference between the hunting and the target guns even though they are all "A400". I really wish one was an A401 or something with a slightly different nomenclature. They obviously have different purposes in mind with their builds even though there is overlap. While all are A400 they chamber different rounds 3" vs 3.5" hulls and have different KO designs to reduce felt recoil.
The target guns are optimized for shorter hulls and lower recoil loads while the hunting versions are optimized for typical heavier hunting loads. Sure there is overlap but each is optimized for a different purpose. On Beretta's website and literature it does say "it shoots all the 12 gauge ammunition, from 24 to 64 grams"
http://www.beretta.com/en-us/a400-xtreme-unico-camo-max5/ so it's not to much to ask that it does as advertised. So you should be able to shoot 24 gram loads from all of them, but that doesn't mean the extreme version will compete heads up with an xcel version shooting clays as the experience will be a bit different due to design differences in things like the KO.
As vtofa mentioned drilling out the second gas hole does work and is very simple to do. You can start small and drill larger as needed. However before you tackle that yourself, I'd give Coles a call since they are an authorized Beretta repair center. Coles will do what is needed to get your gun to shoot everything from 24 to 64 gram loads which is what Beretta claims for the Extreme. Obviously any mods or changes done by an authorized repair center will not void your warranty so this would be the safe route to take.
Keep in mind the Winchester Super X3 sporting is chambered in 2.75" The SX3 Black Shadow is available in 3" or 3.5". The Realtree version is 3.5". So these different models are also optimized for different purposes as well and you might need to"re-tune" these as well if you aren't using them for the optimized purpose.